In recent years, as semiconductor processes become finer, transfer patterns for use in photolithography of semiconductor processes have rapidly become finer. In the next generation, micro-fabrication at 70 nm to 45 nm, and further, micro-fabrication at 32 nm or less would be demanded. In order to meet the demand for, for example, micro-fabrication at 32 nm or less, it is expected to develop an exposure apparatus in which a system for generating extreme ultraviolet (EUV) light at a wavelength of approximately 13 nm is combined with a reduced projection reflective optical system.
Three types of EUV light generating systems have been proposed, which include an LPP (laser produced plasma) type system using plasma generated by irradiating target material with a pulse laser beam, a DPP (discharge produced plasma) type system using plasma generated by an electric discharge, and an SR (synchrotron radiation) type system using synchrotron radiation.    Patent Document 1: International Publication No. 2010/0117861 A    Patent Document 2: US Patent Application Publication No. 2013/0256136 A    Patent Document 3: US Patent Application Publication No. 2010/0294958 A